This study investigates an expanded paradigm to improve health outcomes for adults with knee osteoarthritis (OA), through changes in the low end of the physical activity spectrum where much of the knee OA population resides. Our recent study found almost half of adults with knee OA were inactive, as defined by the US Department of Health and Human Services (DHHS) (i.e., no sustained 10 minute periods of moderate-to- vigorous intensity physical activity during a week) based on objective accelerometer measurement of physical activity. It is these inactive people who will be the primary focus of this study. This renewal will investigate predictors and health outcomes related to transitions from and to physical inactivity. It will also to identify evidence-based physical activity metrics and thresholds needed to maintain function and reduce disability. The study builds on our current funded work from the Osteoarthritis Initiative (OAI), a prospective, observational, cohort study of adults with or at high risk for knee OA that encompasses the full spectrum of knee OA disease severity. This proposal will utilize valuable objective longitudinal physical activity data measure by accelerometer on a subsample from the OAI national cohort (n=2015) collected by the parent study to accomplish the following: Technical Aim. Process accelerometer output into usable physical activity measures and determine inactivity/activity transition status for each person. Aim 1. Evaluate the relationship between a transition from inactivity to activity (T0 toT1) with initia and future changes in disability and function, controlling for baseline total activity, descriptive factors, and modifiable factors. This aim is relevant to clinical practice Aim 2. Examine among inactive adults at baseline the relationship between the change in total activity (T0 toT1) with initial and future changes in disability and function, controlling for baseline total activity, descriptive factors, and modifiable factors. This aim is relevant to the design of public health interventions. Aim 3. Identify predictors of becoming inactive. This aim is relevant to public policy design. Aim 4. Develop evidence-based physical activity metrics to predict short-term (T0 toT1) function and long- term (T0 toT2) disability outcomes among adults at high risk or with knee OA (both active and inactive at baseline). This aim is relevant to refining physical activity public health recommendations. Findings from this proposed study have important public health implications for the design of future physical activity intervention programs to improve quality of life among the 21 million U.S. adults who have knee OA.